Force DHCP : Enable this option to force clients to obtain a valid IP address from DHCP within the specified number of seconds. This prevents clients configured with a static IP address from connecting to the WLAN. Additionally, if a client performs Layer 3 roaming between different subnets, in some cases the client sticks to the former IP address. This mechanism optimizes the roaming experience by forcing clients to request a new IP address.
It states it should also prevent staticly addressed devices connecting as well- even if not roaming between subnets.
You make a good point- but I'm still not sure why the feature should have the impact I am seeing (within a subnet).